In ‘two-component” curable polyurethane adhesives, one component is an “isocyanate group containing component,” and the other component is a component that contains one or more compounds with plural active hydrogens. The “two-component adhesive” is prepared by mixing the two components together, and the mixture is then applied to two or more substrates. Some “two-component” adhesive formulations contain one or more monomeric, aromatic isocyanate compounds, which are usually di-functional, and one or more polyols. Such formulations are capable of undergoing chemical reactions that form urethane polymers, and these reactions are typically useful curing reactions. The monomeric, aromatic isocyanates react to form polymers and/or crosslinks that strengthen the adhesive bond.
These polyurethane adhesives are sometimes used to hold together two or more substrates, to form a bonded assembly for use in food containers. Even though the adhesive is cured, some monomeric isocyanate compounds may be present in the bonded assembly. The presence of monomeric isocyanate compounds is considered to be undesirable, since these compounds are considered to be both toxic and reactive. Also, these compounds are capable of reacting with water to form amines. For example, excess monomeric isocyanate may react with water (for example, trapped water formed during the coating of the adhesive, or water from an atmosphere under high humidity) to form a primary amine (see FIG. 1). Such amines are considered to be undesirable. Among such amines, primary aromatic amines (PAAs) are considered especially undesirable. For food containers, it is desired that contact between the container and the food does not result in appreciable amounts of PAAs in the food.
Whether a bonded assembly contributes to the presence of PAAs in food is generally tested by exposing the bonded assembly to dilute acetic acid for a certain test period. The dilute acetic acid acts as a “simulant” (that is, it simulates the action of food). During the test period, PAAs present in the bonded assembly may migrate into the simulant. Also during that time, monomeric aromatic isocyanate compounds in the material, under test, may also migrate into the simulant, and react with the simulant to form a PAA.
Subsequently, the simulant may be analyzed for the total concentration of all PAAs. That concentration is herein called the “PAA level.”
It is desirable that a bonded assembly has low PAA level. In the past, it was common for a bonded assembly, made using a “two-component” polyurethane adhesive, to have an appreciable amount of monomeric aromatic isocyanate present, even after the formation of the bonded assembly, and the curing reaction of the adhesive, were considered complete. Such a bonded assembly typically had a high PAA level.
In the past, one approach to providing a bonded assembly with a low PAA level was to placed the manufactured assembly in storage, prior to the use of the bonded assembly in a food packaging application. The concentration of monomeric isocyanate will normally decline, while the bonded assembly is in storage. It is thought that water in the atmosphere diffuses into the adhesive and reacts with isocyanate groups. These reactions result in formation of PAA which may further react with other “isocyanate group-containing molecules” to form relatively innocuous urea-type compounds. Therefore, as the isocyanate groups react with water, the amount of monomeric, aromatic isocyanate decreases, and the PAA level also decreases. In the past, storage times of 14 days, or more, have often been required, before the PAA level became acceptably low. There is a need amongst manufacturers of food packaging to reduce the level of PAA to acceptably low levels, within a short period of time, to eliminate long storage times. Thus, there is a need for new forms of food packaging that will effectively scavenge such PAAs.
JP2010-264677 discloses a laminate that has an adhesive layer and a sealant layer sequentially provided on a plastic base material. The adhesive layer contains an isocyanate compound (85 wt % or more) having two or more isocyanate groups and a polydimethyl-siloxane compound (0.01-0.5 wt %). The laminate is used for material for packaging foodstuffs and pharmaceuticals.
JP2010-36413A discloses a laminate having a three-layered structure comprising the following: a layer (A) containing a resin composition containing an olefin polymer and a tertiary polyoxyethylene alkylamine; layer (B) containing polyurethane adhesive and an isocyanate-type adhesive; and layer (C) containing a base material. The laminate film is disclosed as useful for a packaging film for foodstuffs and other materials.
U.S. Pat. No. 6,607,831 discloses a multi-layered article comprising a first layer of a thermoset polyurethane, and a second layer of a polymeric composition, which is bonded to the first layer. The polyurethane has available isocyanate groups prior to the application of the second layer. The second layer is applied onto the first layer in a pre-polymeric or polymeric state, wherein the material has carboxyl groups and a cross-linking agent.
Additional films, laminates and/or compositions are disclosed in the following references: JP2002316396A, JP03918404B2, JP2001152127A, JP04496564B2, JP04407144B2, JP03959967B2 (Abstract), JP03918404B2, JP2000167973A, JP2009142997A, JP03780741B2, JP11000978A, JP3086539A, JP2011016232A, JP2006021530A, JP04402414B2, US20040116643, US20020103284, US20100010156, US20100093942, U.S. Pat. No. 7,101,624, U.S. Pat. No. 7,097,890, U.S. Pat. No. 7,241,481, U.S. Pat. No. 7,368,171, U.S. Pat. No. 7,071,280, U.S. Pat. No. 5,654,061, U.S. Pat. No. 5,047,272, WO 02/16221, WO 97/03821, WO 08/079784, WO 08/080111 and EP1559746A1.
However, as discussed above, there remains a need for new forms of food packaging that will effectively scavenge PAAs. These needs have been met by the following invention.